Lithium silicate glass ceramic for fabrication of dental appliances

ABSTRACT

The present invention relates to a method of fabricating an improved lithium silicate glass ceramic and to that material for the manufacture of blocks for dental appliances using a CAD/CAM process and hot pressing system. The lithium silicate material has a chemical composition that is different from those reported in the prior art with 1 to 10% of germanium dioxide in final composition. The softening points are close to the crystallization final temperature of 800° C. indicating that the samples will support the temperature process without shape deformation.

RELATION TO CORRESPONDING APPLICATIONS

This application is a continuation-in-part of application Ser. No.12/592,825 filed Dec. 3, 2009 which, in turn, is a continuation-in-partof application Ser. No. 12/283,472 filed Sep. 12, 2008 which, in turn,is a continuation-in-part of application Ser. No. 12/082,576 filed Apr.11, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithium silicate glass ceramicmaterial and a process for fabricating that material for the manufactureof blocks and subsequent fabrication of single crowns with the aid of aCAD/CAM process and hot pressing. The invention relates to an improvedversion of such glass ceramic containing germanium dioxide to make itmore castable, with higher density, and with higher flexural strengththan the lithium disilicate glass ceramic free of germanium dioxide.

2. Background Art

There are many products available that employ lithium disilicate glassceramic covered by several U.S. patents. Some of these patents claim aprocess for the preparation of shaped translucent lithium disilicateglass ceramic products from a mixture of basic components (SiO₂, Al₂O₃,K₂O, Li₂O, plus pigments and fluorescent oxides). The thermodynamicsolid-liquid equilibrium of the system consisting of lithium oxide(Li₂O) and silicon dioxide (SiO₂) has been extensively studied evenbefore that material was used as a dental ceramic (1-3, 5-6).

For those skilled in the art this experimental solid-liquid equilibriumcan explain with an extraordinary simplicity how different glassceramics can be obtained using the same two components when they arecombined in different proportions. The same solid-liquid equilibriumshows what type of stable crystal is produced as a final product ofcrystallization when a specific mix composition of the two componentsare blended, melted, and crystallized to achieve the final product.

The crystallographic data for intermediate crystal compounds in theLi₂O—SiO₂ system is given by the Landolt-Bomtein tables. The followingare the types of crystal compositions possible in the Li₂O—SiO₂ system:Li₈SiO₆, Li₄SiO₄ or lithium orthosilicate monoclinic and orthorhombic;Li₆Si₂O₇, Li₂SiO₃ or lithium silicate; Li₂Si₂O₅ or lithium disilicatemonoclinic and orthorhombic; and Li₂Si₃O₇ lithium trisilicate.

Thus when the silicon dioxide to lithium oxide molar ratio (SiO₂/Li₂O)is greater than or equal to two, meaning two moles of SiO₂ are mixedwith one mol of Li₂O, the crystallized glass ceramic product will bemainly lithium disilicate (SiO₂/Li₂O). This molar ratio of two isequivalent to a molar composition of lithium oxide in the mixture of 33%(67% as SIO₂). When the same molar relationship is below 2.0, (e.g. 1.7)only lithium silicate crystals are produced (Li₂O.SiO₂). The lithiumoxide molar composition for a ratio of 1.7 is equivalent to 37% molar(63% SiO₂). The type of resulting crystal due to the specificcomposition ratios gives to the glass ceramic its own distinguishablechemical and physical properties. Surprisingly, the same behavior isobtained if these two main components (silicon dioxide and lithiumoxide) maintain their molar ratio below two even if they are mixed withother oxides as additives and modifiers. The other common oxides mixedare aluminum oxide, potassium oxide, calcium oxide, zirconium oxide andcoloring oxides that are incorporated into the glass matrix and give theglass ceramic its final color and translucency.

Due to the final composition of this invention using a molar ratio ofSiO₂/Li₂O between 1.7 to 1.9, the only phase present is lithiumsilicate, instead of lithium disilicate, as a main constituent of theglass ceramic as a final product. For instance, a glass ceramic with amolar ratio of silicon dioxide to lithium oxide greater than or equal totwo plus additional oxides will produce, after full crystallization alithium disilicate glass ceramic with a melting temperature of 920° C.and a linear thermal coefficient of expansion of 10.5×10⁻⁶/° C. as afinal product and composition. In addition, during the production ofthis type of glass ceramic the cast material is subjected to at leastthree different heat treatments: an annealing cycle for eliminatingaccumulated stresses, a nucleation cycle for the formation of lithiummetasilicate or unstable lithium silicate, and finally a third thermalcycle to convert the unstable lithium silicate or metasilicate into astable lithium disilicate. This is clearly shown in the following USpatents:

Examples of those types of glass ceramics are claimed in Barret et al inU.S. Pat. No. 4,189,325 which discloses a lithium silicate glass ceramicwhere the raw materials are blended, melted at 1315° C. and held for 24hours for homogenization, fritted and crushed, melted again and castinto preheated molds. They disclose a composition of silicon dioxide tolithium oxide molar ratio of two, producing a dental ceramic composed oflithium disilicate.

U.S. Pat. No. 4,480,044 to McAlinn discloses a glass ceramic formulationwhere the lithium silicate glass ceramic in their intermediate processstage has a thermal expansion of 13×10⁻⁶/° C. and the lithium disilicatehas a thermal expansion of 11.4×10⁻⁶/° C. They disclose a machinablelithium disilicate glass ceramic with a percentage of silicon dioxide of79.8%.

U.S. Pat. No. 4,515,634 to Wu et al discloses a castable glass ceramiccomposition useful as a dental restorative material. The components areblended and melted at 1400 to 1450° C., then quenched in water, dried,milled to a powder, and melted again at 1400° C. for 4 hours. Then themelt is cast into copper molds and transferred to the annealing process.The castable glass ceramic of the invention is lithium disilicate with asilicon dioxide to lithium oxide molar ratio of two, equivalent tosilicon dioxide weight composition of 65%-74.7% and lithium oxide weightcomposition of 14.8-16.4%.

U.S. Pat. No. 5,219,799 to Beall et al discloses a lithium disilicateglass ceramic with silicon dioxide weight composition of 65%-80% andlithium oxide compositions of 8.0-19.0%. The blended raw materials aremelted at 1450° C. for 16 hours and then poured into steel molds andannealed at 450° C.

U.S. Pat. No. 5,744,208 to Beall et al describes a lithium disilicateglass ceramic with silicon dioxide weight composition of 75%-95% andlithium oxide weight composition 3-15%. The raw materials are blended,and then melted in the range of 1450-1600° C. for about 6-10 hours. Theglass is then poured into steel molds. The glass is then annealed,nucleated and crystallized to produce lithium disilicate glass ceramicin the range of 500° C. to 850° C.

U.S. Pat. No. 5,968,856 to Scheweiger et al discloses a lithiumdisilicate glass ceramic with compositions of silicon dioxide weightbetween 57%-80% and lithium oxide composition 11-19%. The components areblended and melted at 1500° C. for one hour and then quenched, dried,milled, dry pressed and sintered to form blanks. The compositionrequires the addition of lanthanum oxide to improve the flow properties,control the crystal growth and eliminate the strong reaction of thematerial with the investment material used.

U.S. Pat. No. 6,514,893 to Scheweiger et al discloses a lithiumdisilicate glass ceramic with silicon dioxide composition of 57%-75%weight and lithium oxide composition 13-19% weight and also containinglanthanum oxide. The components are blended and fused into granulatesand comminuted to a powder. Coloring oxides are then added, and theceramic is pressed and heat treated.

U.S. Pat. No. 6,455,451 to Brodkin et al discloses a lithium disilicateglass ceramic with silicon dioxide composition of 62%-85% weight andlithium oxide composition 8-19% weight. They disclose a method of makingthe lithium disilicate by melting the components at 1200 to 1600° C.,followed by a quench, drying, and a heat treatment to form the glassceramic, followed by comminuting to a powder, compacting and sinteringto a blank and pressing to form the restoration.

U.S. Pat. No. 6,517,623 to Brodkin et al discloses a lithium disilicateglass ceramic pressable where the components are melted in the range of1200 to 1600° C., quenched, heat treated, comminuting the glass ceramicto a powder, and then compacting the powder to a starting blank beforesintering the blank or the restoration.

U.S. Pat. No. 6,606,884 to Scheweiger et al describes a lithiumdisilicate glass ceramic where the components are mixed and melted at1200 to 1650° C., followed by pouring the glass into water, milling andcompacting, and placing the blank in a heat treatment to sinter.

U.S. Pat. No. 6,802,894 to Brodkin et al shows a lithium disilicateglass ceramic with a silicon dioxide weight composition of 62%-85% andlithium oxide weight composition 8-19%. The components are mixed, meltedat 1200 to 1600° C., and cast. The resulting glass is annealed at arange of 300 to 600° C., followed by subjecting the glass to a heattreatment from 400 to 1100° C.

U.S. Pat. No. 6,818,573 to Petticrew discloses a lithium disilicateglass ceramic with a silicon dioxide composition of 60%-80% weight andlithium oxide composition of 8-17% weight. The components are blended,melted, quenched, heat treated, milled to a powder, dry pressed, and hotpressed into the desired restoration.

U.S. Pat. No. 7,316,740 to Scheweiger et al claims a lithium silicateglass ceramic with silicon dioxide weight compositons of 64 to 73% andlithium oxide weight compositions of 13 to 17%. The lithium disilicatefinal product is demonstrated by means of a XRD pattern (FIG. 6) and DSCphase transformation curve from lithium metasilicate to lithiumdisilicate (FIG. 2). The DSC diagram shows the change in energy from thestage of metasilicate to disilicate, which is only necessary if lithiumdisilicate is desired to be the crystal phase used as a final product.

U.S. Pat. No. 7,452,836 to Apel et al discloses a lithium silicate glassceramic with silicon dioxide composition of 64%-75% weight and lithiumoxide composition of 13-17% weight producing lithium disilicate as afinal product. They also describe a glass ceramic with a molar ratio ofsilicon dioxide to lithium oxide of at least 2.3.

U.S. Pat. No. 7,867,930 to Apel et al shows a lithium silicate glassceramic with silicon dioxide composition of 64%-75% weight and lithiumoxide composition of 13-17% weight producing lithium disilicate as afinal product.

U.S. Pat. No. 7,871,948 to Apel et al describes a lithium silicate glassceramic with silicon dioxide composition of 64%-75% weight and lithiumoxide composition of 13-17% weight, producing lithium disilicate as afinal product. The glass of the starting material is subjected to aninitial heat treatment form lithium metasilicate or unstable lithiumsilicate and then goes through a second heat treatment to convert thelithium metasilicate to a lithium disilicate.

U.S. Pat. No. 7,867,931 to Apel et al discloses a lithium silicate glassceramic with silicon dioxide composition of 64%-75% weight and lithiumoxide composition of 13-17% weight producing lithium disilicate as thefinal product. They also describe a glass ceramic with a molar ratio ofsilicon dioxide to lithium oxide in the range of 2.3 to 2.5.

U.S. Pat. No. 8,042,358 to Schweiger et al discloses a lithium silicateglass ceramic with silicon dioxide composition of 65%-70% weight andlithium oxide composition of 14-16% weight producing lithium disilicateas the final product. In their specific process the raw materials suchas carbonates, oxides and phosphates are prepared and melted in therange of 1300-1600° C. for 2 to 10 hours. They explain that in order toobtain a particularly high degree of homogeneity the glass melt obtainedmay be poured into water to form glass granulates and the glassgranulates obtained are melted again.

For those skilled in the art it is understandable that the lithiumoxide—silicon dioxide system has been extensively studied and severalpatents for dental glass ceramics have been granted in the last fewyears. However all the research so far falls in the range where lithiumdisilicate is formed as a final product and none of the references abovedisclose lithium silicate glass ceramic as a final product. For thoseskilled in the art it is evident that the type of crystal produceddepends exclusively on the molar ratio of silicon dioxide to lithiumoxide in the glass ceramic and not the additives or modifiers added tothe mixture. This molar ratio controls the type of crystal formed in thefinal composition and furthermore give its name to the final glassceramic.

Most of the existing patents in the dental field use the same basiccomponents. The present invention uses germanium dioxide as afundamental part of the formula. This oxide is broadly used in glasspreparation for its good optical properties. The oxide has been wellstudied and has positive effects compared to common silicon glasses. Ithas been found that the addition of germanium oxide produces a melt withlow viscosity, which facilitates the castability of the process andincreases the thermal expansion and the refractive index of theresulting lithium silicate glass ceramic. More importantly, the additionof germanium dioxide increases the final density of the glass resultingin higher values of flexural strength than the lithium disilicateglasses free of germanium dioxide. U.S. Patent Application PublicationNo. 2004/0197738 to Ban et al discloses a process to make dental frameof zirconium-ytrium sintered ceramics and they describe dental porcelainwith germanium oxide as a joint component different than the zirconiumyttrium oxide frame. However germanium oxide is not used as a componentof the framework ceramic network. It is used only in formulation of theceramic joint and is just a part of a series of other oxides that can bejoined to the framework material.

Due to the low silicon dioxide to lithium oxide molar ratio of 1.7 ofthe present invention, equivalent to 37% molar of lithium oxide (63%silicon dioxide) the ceramic has a lower melting point compared to theglass ceramic of the prior art. In addition, this new glass ceramiccontains the lowest silicon dioxide weight percent compared to all ofthe noted prior art. Therefore, due to this specific composition oflithium oxide in the mixture, the type of resulting crystal aftercrystallization (lithium silicate) gives to the glass ceramic its ownchemical and physical properties, which makes it completelydistinguishable from the prior glass ceramics listed above. Due to thisdistinguishable composition, the present glass ceramic has a lowermelting temperature and can be made even lower with the addition ofgermanium oxide. Germanium oxide replaces silicon dioxide in the glassnetwork, causing it to have a negative effect on the resulting meltingpoint compared to a glass ceramic containing only silicon dioxide. Thusthe processing and optimal melting temperature is in the range of 1100°C. to 1200° C. instead of 1200° C. to 1650° C. of the U.S. patents citedabove and specifically compared to U.S. Pat. No. 6,514,893 to Schweigeret al. The glass ceramics listed in the prior art patents cannot be castin the range of 1100° C. to 1200° C. because they are too viscous due totheir high silicon dioxide content, therefore the processes disclosed inprior art patents with higher melting temperatures should be used. Thepresent process will result in a more economical production because theenergy employed for melting the glass is considerably lower and thereare lower energy loses by radiation compared to the Schweiger process.

In addition to having a process with lower energy consumption, anothersignificant improvement of the inventive process is related to themixing and reaction of the components. In all of the previously citedprior art patents, the mix of the components is blended and melted at1400 to 1650° C. and then cast or quenched in water. The quenched glasspowder is dried, milled, and melted again in order to improve thehomogeneity and the quality of the product. Surprisingly, it was foundthat the first melting and casting process can be avoided if oneperforms a calcination process on the mixture of raw materials to atemperature in the range of 700 to 800° C. without melting thecomponents. At this stage, all the raw materials in the form of salts(like lithium carbonate as the source of lithium oxide, calciumcarbonate as the source of calcium oxide, and di-ammonium phosphate asthe source of phosphorous oxide) are decomposed, eliminating gases suchcarbon dioxide and ammonia, producing a ceramic powder free of gases.After cooling down, the calcined mix is milled again, producing ahomogeneous powder with a very small particle size. The final step ismelting and casting in the range of 1100° C. to 1200° C., resulting in ahomogeneity of all the components. In addition, by eliminating the gasesduring the calcination process, the glass cast becomes bubble free,making this a significant advantage over the processes described in theprior art.

The present invention is also unique compared to those listed in theprior art due to its composition. The use of a low melting temperatureis only possible with the present glass ceramic because of the lowcontent of silicon dioxide and the high content of lithium oxide. Thistranslates to a molar oxide ratio (SiO₂/Li₂O) below 2.0, (i.e., 1.7) inwhich only lithium silicate crystals are produced (SiO₂—Li₂O). Inaddition to the composition, the present invention implements a processfor a glass ceramic that produces a homogeneous product and that can beused only with the specific formulation. This process cannot be usedwith the other listed glass ceramics due to the lower operatingtemperatures.

The present invention emphasizes that in the inventive glass ceramic thesilicon dioxide and lithium oxide molar ratio content (SiO₂/Li₂O) isless than 2, specifically the oxide molar ratio is preferably about 1.7.This is specifically equivalent to 63% molar of silicon dioxide and 37%molar of lithium oxide, and specifically equivalent in the overallformulation of about 56% weight percent of all of the glass ceramic assilicon dioxide and 16.0% weight percent as lithium oxide and theremaining 28% composed of the oxide additives and modifiers. In all ofthe glass ceramic, only lithium silicate (Li₂O—SiO₂) crystals areproduced as the final crystal phase product. During the heating processof the glass, the first crystals formed are stable lithium silicate andthey remain stable through the end of the growing process. This meansthat there is no need for a third thermal process for producing thefinal crystal of lithium silicate making this an additionalcharacteristic unique to the present invention. This new ceramic hassoftening temperature of about 700 to 800° C. and a linear thermalcoefficient of expansion of about 12 to 12.5×10⁻⁶/° C. as a finalproduct and a composition yielding completely different chemical andphysical properties compared to the prior art. This is easilydemonstrated in commonly assigned U.S. patent application Ser. No.12/592,825, paragraph [0012], FIG. 1 showing a XRD pattern diffractionwhere only lithium silicate crystals are present in the final productand paragraph [0013] where the glass ceramic has a percentage linearchange of 0.55% measured at 500° C. and an equivalent coefficient ofthermal expansion of 11.5×10⁻⁶/° C.

The following is a list of non-patent references noted herein:

-   1. MARCUS P. BOROM et al, Strength And Microstructure In Lithium    Disilicate Glass-Ceramics, J. Am. Ceram. Soc., September-October    1975, Vol. 58, No. 9-10, pp 385-391. The authors prepare lithum    disilicate glass ceramics and measured the differences between the    thermal expansion of the lithium disilicate with a value of    13×10⁻⁶/° C. and lithium silicate with a value of 11.4×10⁻⁶/° C.    After the heat treatment above 800° C. the only phase present is    lithium disilicate for a glass ceramic composition of 71.8% of    silicon dioxide and 12.6 of lithium oxide.-   2. R. A. EPPLER, Glass Formation And Recrystallization In The    Lithium Metasilicate Region Of The System Li₂O—Al₂O₃—SiO₂, J. Am.    Ceram. Soc., February 1963, Vol. 46, No. 2, pp 97-101.-   3. F. A. HUMMEL, Thermal Expansion Properties Of Some Synthetic    Lithia Minerals, J. Am. Ceram. Soc., August 1951, Vol. 34, No. 8, pp    235-239.-   4. LANDOLT-BÖRNSTEIN (LB), Group IV Physical Chemistry—Phase    Equilibria, Crystallographic And Thermodynamic Data For Intermediate    Compounds in the Li₂O—SiO₂ System.-   5. S. CLAUS et al, Phase Equilibria In The Li₄SiO₄—Li₂SiO₃ Region Of    The Pseudobinary Li₂O—SiO₂, Journal of Nuclear Materials, Vol. 230,    Issue 1, May 1996, pp 8-11.-   6. HERMAN F. SHERMER, Thermal Expansion Of Binary Alkali Silicate    Glasses, Journal of Research of the National Bureau of Standards,    Vol. 57, No. 2, August 1956. The author prepares lithium silicate    glasses with silicon oxide and lithium oxide molar ratio below 2.0    being lithium silicate with thermal expansion between 12 and    14.77×10⁻⁶/° C. There is no lithium disilicate using this chemical    molar composition.

SUMMARY OF THE INVENTION

The present invention relates to preparing an improved lithium silicateglass ceramic for the manufacture of blocks for dental appliancefabrication using a CAD/CAM process and hot pressing. The lithiumsilicate material has a chemical composition that is different fromthose reported in the prior art, especially because of the use ofgermanium dioxide in the formulas and its low silicon dioxide content.The softening points are close to the crystallization final temperatureof 800° C. indicating that the samples will support the temperatureprocess without shape deformation.

The initial components are chemical precursors, specifically aluminumhydroxide for aluminum oxide, boric acid for boron oxide, lithiumcarbonate for lithium oxide, ammonium hydrogen phosphate or calciumphosphate for phosphorus pentoxide, zirconium silicate or yttriumstabilized zirconia for zirconium oxide, calcium carbonate for calciumoxide, lithium fluoride for lithium oxide and fluoride, and potassiumcarbonate for potassium oxide. The remaining elements are single oxideprecursors of silicon, cerium, titanium, tin, erbium, vanadium,germanium, samarium, niobium, yttrium, europium, tantalum, magnesium,praseodymium, and vanadium oxides.

The components are carefully weighed and then mechanically blended usinga V-cone blender for about 5 to 10 minutes. Then in order to achieveuniform particle size of the components, the mixture undergoes a ballmill process for two hours. The powder obtained is put into largealumina crucibles and undergoes calcination to 800° C. for about 4hours. In this stage the carbonate precursors, lithium carbonate,calcium carbonate, potassium carbonate, decompose releasing carbonic gasand producing the corresponding pure oxides, lithium oxide, calciumoxide and potassium oxide, respectively. In the same process the otherchemical precursors, ammonium phosphate, aluminum hydroxide and boricacid also release nitrogen gases and water producing the correspondingpure oxides, phosphorous pentoxide, aluminum oxide and boron oxide,respectively. At this stage of calcination the original powder mix losesapproximately 25% of its original weight due to the evaporation losses.Also, the first reactions between the pure oxides are taking place inthis stage but there is never any melting of the components and noreaction takes place with the alumina crucible. After cooling down, theblend of components undergoes ball milling again, producing ahomogeneous, gas free, fine powder with a particle size below 30microns. The calcined powder can be safely stored in plastic containersfor extended periods of time without any gas release and can be usedanytime for the next step of the process.

In the final stage of the process the calcined powder is melted in aplatinum crucible at a temperature of 1200° C. with a holding time ofabout 2 hours before casting. The melt with the appropriate viscosity iscast continuously over graphite molds. Surprisingly, the glass cast isbubble free due to the prior elimination of the gases during thecalcination step. This constitutes a significant advantage over theprocesses described in the prior art. Due to the calcination processstep, there is no need for a second re-melting process for improvinghomogeneity. The glass cast is then subjected to an annealing stepfollowed by an intermediate crystallization step or a fullcrystallization step depending on what is desired as a final product.

Due to the specific molar ratio of silicon dioxide and lithium oxide(1.7/1) used in the present invention, the only preferred crystalstructure formed is lithium silicate (SiO₂Li₂O) in the intermediate orfull crystallized product. Surprisingly it was found that in thisinvention, the crystal growth process can be momentarily stopped at anytemperature interval between the ranges of 350° to 800° C. and then thecrystal can continue growing by heating it again to reach the optimalsize at 800° C. Above 800° C. the sample starts melting and the reverseprocess of dissolving the crystals in the glass matrix takes place.

Thus in the present invention, the intermediate crystallization processstep is easily controlled by stopping the heating process at 600° C. andcooling down to room temperature. It can then be heated again to 800° C.for achieving the full crystallized product. Thus if one takes theintermediate block material of lithium silicate, after the thermal heatprocess from room temperature to 600° C., it can be milled to a dentalrestoration using conventional CAD/CAM devices and then it can be heatedup again to 800° C. continuing towards maximum crystal growth andachieving the optimal physical properties. Surprisingly, the sameformulation, after a thermal process from room temperature to 800° C.,can be easily hot pressed in the range of 800-840° C. using conventionalall ceramic dental investments and commercial press furnaces (i.e WhipMix Pro-Press 100). For the hot press process, the dental restoration ismilled in a wax block, followed by investing the wax pattern usingcommercial all ceramic investments. After firing the investment, the waxis burned out, allowing the cavity of the restoration to becomeavailable to fill with the ceramic. After hot pressing the restorationachieves the optimal physical properties.

The same formulation produces the same lithium silicate crystallinephase through all the thermal process steps and the dental restorationcan be optimally achieved by using either CAD/CAM or hot presstechniques. Being able to achieve this with the same formulation is aunique and advantageous characteristic over the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the present invention, aswell as additional objects and advantages thereof, will be more fullyunderstood herein after as a result of a detailed description of apreferred embodiment when taken in conjunction with the followingdrawings in which:

FIG. 1 is an XRD diffraction pattern of a sample of the invention afterthe intermediate crystallization step (from room temperature to 600° C.)showing the presence of lithium silicate as a main constituent phase inthe glass ceramic composition;

FIG. 2 is an XRD diffraction pattern of a sample of the invention afterthe full crystallization step (from room temperature to 800° C.) showingthe presence of lithium silicate as a main constituent phase in theglass ceramic composition. Because the molar ratio of SiO₂/Li₂O isbetween 1.7 to 1.9, the crystallized phase of the final material showsthe presence of only lithium silicate and no lithium disilicate;

FIG. 3 is an XRD diffraction pattern of a sample of this invention afterhot pressing in the interval of 800° C. to 840° C. showing the presenceof lithium silicate as a main constituent phase in the glass ceramiccomposition. Because the molar ratio of SiO₂/Li₂O is between 1.7 to 1.9,the crystallized phase of the final material shows the presence oflithium silicate and no lithium disilicate; and

FIG. 4 is a graphical illustration of a dilatometric measurement of asample of the invention resulting from full crystallization. Thesoftening temperature of the intermediate step is lower than thetemperature after full crystallization. This is due to the crystalgrowth after heating the glass in the intermediate stage from roomtemperature to 800° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The prior art materials are based on the formation of lithium disilicatematerials. A principal object of the present invention is to prepare acontrolled lithium silicate glass ceramic using in the formulation aspecific silicon dioxide and lithium oxide molar ratio with excellentphysical properties for manufacturing dental restorations. The glassmaterial subjected to a heat treatment produces an optimal lithiumsilicate crystal forming a glass ceramic product with outstandingmechanical properties, excellent optical properties, a very goodchemical solubility, little contraction and high flexural strengthvalues.

The lithium silicate of the present invention preferably comprises thefollowing components and compositions:

weight % composition Component minimum maximum SiO₂ 53.0 57.0 Al₂O₃ 3.05.0 K₂O 3.0 5.0 CaO 0.0 1.0 B₂O₃ 0.0 2.0 CeO₂ 0.0 1.0 MgO 0.0 1.0Fluorine 0.0 1.0 Li₂O 14.0 17.0 ZrO₂ 4.0 5.0 TiO₂ 0.0 3.0 P₂O₅ 2.0 3.0SnO 0.0 1.0 Er₂O₃ 0.0 2.0 V₂O₅ 0.0 1.0 GeO₂ 0.5 8.0 Ta₂O₅ 0.0 3.0 Sm₂O₃1.0 6.0 Pr₂O₃ 0.0 1.0 Eu₂O₃ 0.0 2.0 Y₂O₃ 0.0 5.0 Nb₂O₅ 0.0 1.0

The invention is explained in more detail below with the followingexamples:

The sample preparation and its elemental oxide composition are listed inTable 1.

TABLE 1 Components % weight. Example 1 Example 2 Example 3 Example 4Example 5 SiO₂ 55.03 56.19 56.21 56.21 53.88 Al₂O₃ 4.09 4.18 4.18 4.183.11 K₂O 4.42 4.52 4.52 4.52 3.44 CaO 0.94 0.96 0.96 0.96 0.00 B₂O₃ 1.581.61 1.61 1.61 0.00 CeO₂ 0.21 0.65 0.34 0.41 0.63 MgO 0.22 0.23 0.230.23 0.00 Fluorine 0.49 0.50 0.50 0.50 0.00 Li₂O 15.81 16.14 16.15 16.1514.81 ZrO₂ 4.70 4.79 4.80 4.80 4.88 TiO₂ 2.40 0.80 0.80 0.80 0.63 P₂O₅2.52 2.58 2.58 2.58 2.94 SnO 0.22 0.07 0.13 0.12 0.00 Er₂O₃ 0.37 0.760.36 0.21 1.26 V₂O₅ 0.39 0.22 0.26 0.11 0.03 GeO₂ 0.90 0.92 0.92 0.927.75 Ta₂O₅ 0.07 0.15 0.22 0.01 0.00 Sm₂O₃ 2.03 4.09 4.07 4.09 5.71 Pr₂O₃0.03 0.33 0.04 0.00 0.88 Eu₂O₃ 0.00 0.00 0.00 1.25 0.05 Y₂O₃ 3.13 0.110.61 0.36 0.00 Nb₂O₅ 0.46 0.22 0.53 0.00 0.00 TOTAL 100.00 100.00 100.00100.00 100.00 Example 6 Example 7 Example 8 Example 9 Example 10 SiO₂54.08 54.49 56.17 53.49 56.19 Al₂O₃ 3.12 3.86 4.18 3.98 4.18 K₂O 3.454.20 4.52 4.30 4.52 CaO 0.00 0.00 0.96 0.92 0.96 B₂O₃ 0.00 0.00 1.611.53 1.61 CeO₂ 0.95 0.64 0.00 0.20 0.62 MgO 0.00 0.00 0.23 0.22 0.23Fluorine 0.00 0.00 0.50 0.48 0.50 Li₂O 14.85 15.25 16.15 15.37 16.14ZrO₂ 4.89 4.88 4.80 4.56 4.79 TiO₂ 0.63 0.64 0.80 0.78 0.80 P₂O₅ 2.952.97 2.58 2.45 2.58 SnO 0.00 0.00 0.00 0.00 0.07 Er₂O₃ 1.52 1.28 0.050.16 0.61 V₂O₅ 0.06 0.04 0.00 0.48 0.15 GeO₂ 7.77 7.70 0.92 0.87 0.92Ta₂O₅ 0.00 0.00 2.33 0.00 0.18 Sm₂O₃ 4.82 3.34 1.83 4.90 4.05 Pr₂O₃ 0.900.72 0.00 0.23 0.24 Eu₂O₃ 0.00 0.00 0.05 0.00 0.00 Y₂O₃ 0.00 0.00 2.334.90 0.24 Nb₂O₅ 0.00 0.00 0.00 0.18 0.45 TOTAL 100.00 100.00 100.00100.00 100.00

A particularly preferred lithium silicate material as described in theexamples 1 to 10 comprises 53 to 59 wt % of SiO₂, 14 to 19% wt of Li₂0and 1 to 9% of GeO₂, where after nucleation only lithium silicate isformed and then after complete crystal growth only lithium silicatecrystals are formed.

The lithium silicate material of this invention is preferably producedby a process which comprises the following steps:

-   -   (a) A mix of the precursors of the final components of the table        1, are blended together for 10 to 30 min until a mechanical mix        is obtained.    -   (b) The mix is ball milled dry or wet using zirconia media for        about 1 to 2 hours to homogenize the components and achieve        almost the same particle size in all the components.    -   (c) The sample is calcined at 800° C. for about 1 to 4 hours in        order to decompose the precursors to their primary oxides and        eliminate any possibility of formation of gas after the process.    -   (d) Ball-mill the sample of step (c) in order to produce a        powder with an average particle size below 30 microns.    -   (e) The powder of step (d) is melted in a platinum crucible at a        temperature between 1100 to 1200° C. for 1 to 2 hours. It is        then poured into cylindrical or rectangular graphite molds and        cooled down to room temperature.    -   (f) The glass ceramic of step (e) is then subjected to an        intermediate crystal growth process at a temperature of from        room temperature to 600° C. for 10 to 60 min. The growth of the        lithium silicate crystals is temporarily stopped for the desired        intermediate size by cooling the glass ceramic to room        temperature.    -   (g) The glass ceramic of step (f) is subjected to a single step        heating cycle from room temperature to 800° C. to achieve full        crystallization.    -   (h) For use in a CAD-CAM milling device, the dental restoration        is made using a block after intermediate process step (f). After        milling, the restoration is heated again from 350° C. to 800° C.        or to full crystallization step (g) where the optimal lithium        silicate crystal growth in the glass ceramic is achieved in a        single step program.    -   (i) For an alternative hot pressing technique, the sample after        [step (g)] is pressed into a dental restoration at a temperature        of 800-840° C., where the optimal lithium silicate crystal        growth in the glass ceramic is achieved.

Coefficient of Thermal Expansion and Softening Point

The percentage linear change vs. temperature was measured using an Ortondilatometer. The coefficient of thermal expansion at 500° C. and thesoftening point were calculated for all the samples. For this purpose arectangular rod of approximately 2 inches long was cast and thensubjected to the intermediate crystallization cycle at 600° C. for 40min. After this process the rod is cut into two parts. One part is usedfor measuring transition temperature, softening point temperature, andcoefficient of thermal expansion of that process step. The second partis fully crystallized at 800° C. for about 10 minutes and is used formeasuring the same properties. It is expected that after thecrystallization step, the softening temperature point increases for thesamples due to the formation of larger lithium silicate crystals. Testresults are displayed in Table 2.

Flexural Strength.

Biaxial flexural strength tests (MPa) were performed following ISO-6872procedures. Ten round samples were cut, grinded gradually and polishedto a mirror finish in the intermediate stage or step (f). The sampleswere then fully crystallized in a single stage program from 350° C. to800° C. for 10 minutes. Then the biaxial flexural strength was measured.For the hot pressing technique the glass ceramic of sample of step (g)is hot pressed into round discs in the interval of 800 to 840° C. Thenthe discs are grinded gradually and polished to a mirror finish, heatedas a simulated glaze cycle, and tested. Test results expressed in MPaare displayed in Table 2.

Chemical Solubility.

A chemical solubility test was performed according to ISO-6872. Tendiscs samples subjected to step (g) are placed in a glass flask with anaqueous solution of 4% (V/V) of acetic acid analytical grade (AlfaAesar). The flask is heated to a temperature of 80+/−3° C. for 16 hours.The change in weight before and after the test is determined and thenthe chemical solubility expressed as μg/cm² is calculated and shown inTable 2.

TABLE 2 Physical Properties of the Lithium silicate glass ceramic.Example Example Example Example Example #2 #3 #4 #5 #8 Softeningtemperature, ° C., 689 618 690 766 711 Intermediate stage at 600° C.Softening temperature, ° C., 727 744 717 789 724 crystallized sample at800° C. Coefficient of expansion, X10⁻⁶/° C. 11.81 12.58 12.27 11.3011.61 Crystallized sample at 800° C. Flexural strength, MPa, 350 +/− 28402 +/− 56 359 +/− 40 365 +/− 60 370 +/− 50 Crystallized at 800° C.Flexural strength, MPa 393 +/− 48 423 +/− 61 523 +/− 39 345 +/− 20 397+/− 57 Hot pressed sample Chemical Solubility, μg/cm² 72 58 65 39 58Crystallized sample at 800° C.

The preferred range composition (in % wt) of this glass ceramic materialis the following:

TABLE 5 Preferred Range of Composition Components weight % compositionComponent minimum maximum SiO₂ 53.5 56.2 Al₂O₃ 3.1 4.2 K₂O 3.4 4.5 CaO0.0 1.0 B₂O₃ 0.0 1.6 CeO₂ 0.0 1.0 MgO 0.0 0.2 Fluorine 0.0 0.5 Li₂O 14.816.1 ZrO₂ 4.6 4.9 TiO₂ 0.6 2.4 P₂O₅ 2.5 3.0 SnO 0.0 0.2 Er₂O₃ 0.1 1.5V₂O₅ 0.0 0.5 GeO₂ 0.9 7.8 Ta₂O₅ 0.0 2.3 Sm₂O₃ 1.8 5.7 Pr₂O₃ 0.0 0.9Eu₂O₃ 0.0 1.3 Y₂O₃ 0.0 4.9 Nb₂O₅ 0.0 0.5

One preferred example of this material has the following specificcomposition:

TABLE 6 Preferred Composition Component Weight % SiO₂ 55.74 Al₂O₃ 4.15K₂O 4.48 CaO 0.95 B₂O₃ 1.60 MgO 0.23 Fluorine 0.50 Li₂O 16.01 ZrO₂ 4.76TiO₂ 0.80 P₂O₅ 2.56 GeO₂ 0.91 Coloring oxides 7.32

Having thus disclosed a number of embodiments of the formulation of thepresent invention, including a preferred range of components, apreferred formula thereof and a preferred fabrication process, thosehaving skill in the relevant arts will now perceive variousmodifications and additions. Therefore, the scope hereof is to belimited only by the appended claims and their equivalents.

1. A lithium silicate ceramic glass made from a composition mixturecomprising: 53% to 57% wt of SiO₂; 14 to 17% wt of Li₂O; 0.5 to 8% wt ofGeO₂; 3.0 to 5.0% wt of Al₂O₃; 3.0 to 5.0% wt of K₂O; 0.0 to 2.0% wt ofB₂O₃; 0.0 to 1.0% wt of CaO; 0.0 to 1.0% wt of Fluoride; 0.0 to 1.0% wtof MgO; 0 to 1.0% wt of CeO₂; 2.0 to 3.0% wt of P₂O₅; and 0 to 10% ofcoloring oxides.
 2. The lithium silicate ceramic glass recited in claim1 wherein said composition mixture also comprises at least one of thefollowing components: ZrO₂, TiO₂, Er₂O₃, V₂O₅, MnO₂, Tb₄O₇, Ta₂O₅,Sm₂O₃, Pr₂O₃, Y₂O₃, Nb₂O₅, SnO, and Eu₂O₃.
 3. A lithium silicate ceramicglass made from a composition mixture comprising: about 56.0% wt SiO₂;about 16.0% wt Li₂O; about 4.5% wt K₂O; and about 4.2% wt of Al₂O₃;about 5.0% wt of ZrO₂, about 5.0% wt of Sm₂O₃ and about % 2.0 wt ofGeO₂.
 4. The lithium silicate ceramic glass recited in claim 3 whereinsaid composition mixture also comprises at least 3.0% wt of each of thefollowing components: P₂O₅, Pr₂O₃ and Y₂O₃.
 5. The lithium silicateceramic glass recited in claim 4 further comprising at least one of thefollowing additional components: TiO₂, SnO, Eu₂O₃, Er₂O₃, Nb₂O₃ andV₂O₅.
 6. The lithium silicate glass ceramic recited in claim 1comprising a molar ratio of SiO₂/Li₂O between 1.7 and 1.9
 7. A method offabricating lithium silicate glass; the method comprising the steps of:(a) blending a mix of precursors; (b) ball milling the mix to homogenizethe components of the mix for 2 to 4 hours; (c) calcining the resultingmix of step (b) for about 4 to 5 hours at a temperature of 700 to 800°C.; (d) ball-milling the calcined mix of step (c) to homogenize thecomponents of the mix for 2 to 4 hours; (e) melting the resulting mix ofstep (d) at a temperature of 1100 to 1200° C. for 3 to 6 hours; and (f)pouring the melt of step (e) into graphite molds to form shaped blanksand cooling such blanks to room temperature.
 8. The method offabricating dental restorations of lithium silicate glass ceramic asrecited in claim 7; the method comprising the additional steps of: (g)heating the blanks from room temperature to 600° C. at 1° C./min, andholding the blanks for 40 to 50 min at 600° C.; (h) milling the blanksof step (g) into dental restorations; and (i) heating the restoration ofstep (h) at temperature of 350° to 800° C. for 10 to 15 minutes.
 9. Themethod of fabricating dental restorations of lithium silicate glassceramic as recited in claim 7; the method comprising the additionalsteps of: (g) heating the blanks from room temperature to 800° C. at 15°C./min for 10 to 40 minutes; and (h) hot pressing the blanks of step (g)into dental restorations at a temperature of 800° to 840° C.
 10. Themethod of claim 8 wherein the final crystalline product is lithiumsilicate.
 11. The method of claim 9 wherein the final crystallineproduct is lithium silicate.
 12. The method of claim 7 wherein one ofsaid precursors in step (a) is germanium oxide
 13. The method offabricating lithium silicate glass recited in claim 7 wherein said mixof precursors of step (a) employs a molar ratio of SiO₂/Li₂O which isbetween 1.7 and 1.9.